Method of making fine mesh metallic screens



Fb. 15 1955 b. J. DONAHUE EIAL 2,702,270

mom for MAKING FINE MESH METALLIC SCREENS Filed June '7, 1952 LE-XHNDER M. RENNIE- 5mm. J. D [manua- United States Patent 2,702,270 METHOD OF MAKING FINE MESH METALLIC SCREENS Daniel I. Donahue, Lancaster, and Alexander M. Rennie, Landisville, Pa uslgnors to Radio Corporation of America, a corporation of Delaware Application June 7, 1952, Serial No. 292,354 8 Claim!- (Cl- 204-11) The resent invention relates to an improved method of making metallic screens of very fine mesh and having a gossamer-like thinness.

Although screens made by the method of the present invention can be used for any desired purpose, one of the applications for which they are particularly designed is that of an electrodein television camera tubes. Screens used in television camera tubes are required to be extremely uniform, be of very fine mesh, capable of being stretched taut without curling, and also be of relattvely high electron permeability and light permeability.

In the past, metal screens of fine mesh have been made by a number of difierent methods. One of these methods was to make the screen of woven wire of extreme thinness. This method has proven generally unsatisfactory because of the difiiculty of carrying it out and because of the non-uniformity of the product made. Such screens also have relatively low light transmission and electron transmission values. Another method formerly used has involved the electro-deposition of metal upon conductive parts of a pattern made with the aid of a light sensitive emulsion. In general, screens made by this method have proven unsatisfactory for about the same reasons as woven wire screens.

Another previous method of making fine mesh metal screens is described in U. S. Patent 2,529,086 issued November 7, 1950, to H. B. Law. By this method, screens can be made which are entirely satisfactory for use in television camera tubes. However, it has been found that the method is difficult to control so as to obtain uniform results. One of the difiiculties in the formerly patented process is that a sputtered metal coating is used, this sputtered metal being deposited from a cathode painted with a suspension of a mixture of the metals, palladium, gold, and bismuth. In practice, it has proven diflicult to coat this suspension uniformly on the cathode in exactly the same way each time and to deposit a sputtered coating in such a way that perfect metal deposits can be made over it. Also, the screen which is made by this method has a considerable tendency to curl and tear at various points unless extreme care is taken in production.

The present invention is an improvement in the method described in the above-mentioned patent. It is much easier to use in factory production and produces screens of greater uniformity and more satisfactory texture. In general, the improved method of the present invention comprises the steps of depositing a very thin coating of wax over the entire surface of a hard ceramic master plate which has previously been provided with a grid work of etched grooves having a spacing such as required in the finished screen, depositing a thin layer of a metal such as palladium, platinum, or gold on top of the wax coating by a sputtering process, removing all of the sputtered metal coating from the raised areas between the etched. grooves, electro-depositing another metal over the sputtered metal coating in the grooves so as to form a grid work or screen of metal, and separating the formed screen from the coated ceramic master plate.

One object of the present invention is to provide an improved method of making fine mesh metallic screens.

Another object of the present invention is to provide a method of making fine mesh screens which are less susceptible to curling and tearing.

Another object of the invention is to provide an improved method of making very fine mesh metallic screens 2,702,270 Patented Feb. 15, 1955 in which the product is more uniform than in previously used processes.

Another object of the present invention is to provide an improved method of making fine mesh screens which is more practical in ordinary factory production.

Another object of the present invention is to provide an improved method of making fine mesh screens which enables the elimination of certain factors which, in the past, have proven very difiicult to control.

Another object of the invention is to provide an improved method of making fine mesh screens which enables the re-use of glass masters formerly discarded because of slight surface imperfections.

. These and other objects will be more apparent and the invention will be more readily understood from the following detailed description and the accompanying drawings of which,

Figure 1 1s a perspective view of a glass master plate or matrix having on one surface a grid work of fine grooves corresponding in number and width to the metal screen which is to be the final product,

Figure 2 is a magnified fragmentary view of part of the screen portion of the master plate of Figure 1,

Figure 3 is a view like that of Figure 2, the portion of the plate shown being coated with a thin film of wax,

Figure 4 is a cross-section view taken along the line 4 i of Figure 3,

Figure 5 is a view similar to that of Figure 4 with a thm coating of sputtered metal laid down over the wax coating,

Figure 6 is a view similar to that of Figure 5 with the sputtered metal removed from the raised areas between I the grooves,

Figure 7 is a view similar to Figure 6 with an additional thickness of a different metal deposited over the sputtered medtal coating within the grooves of the master plate, an

Figure 8 is a fragmentary perspective view of a completed screen made by the process of the present invention.

A preferred example of making a screen in accordance with the improved process of the present invention will now be given.

Example For making screens in accordance with the process of the present invention, it is desirable to start with a spaced grooves 6. Although the number of grooves per inch may vary widely, as for example 200 to 2000 per inch, the plate of the present example has an area 3% inches square provided with 720 grooves per inch both horizontally and vertically, crossing each other at right angles. These grooves must be ruled with extreme uniformity for best results and, although their depth and width are subject to considerable variation depending upon the end use of the product, it has been found suitable to use grooves having a depth of 0.1 mil and a width of 0.4 mil. Although glass is preferred as the supporting plate, other hard ceramics may be used.

The master plate is carefully cleaned so as to remove all surface contamination from within the grooves and the raised areas between the grooves. The cleaned plate is then dipped briefly in a solution of beeswax so as to apply a very thin coating 8 of the wax over the entire plate. This solution can also be applied by other methods, such as spraying, flowing, or brushing. A 75% saturated solution of beeswax in petroleum ether having a boiling point between 30 and 70 C. has been found suitable. Any other volatile solvent for the wax may be used, for example hexane or benzene. With this concentration of wax, it is necessary to make only a single dip of the plate in the solution. If considerably lower concentrations of wax are used, more than one dip may be required. Other waxes may be used equally as well as beeswax. For example, waxes ,such as carnauba, paraffin, and montan can be used. Although some waxes are preferred over others for best results,

the use of a particular wax is not critical and, in general,

any wax may be used. It is desirable to use a highly volatile solvent for applying the wait so that wax coating will dry rapidly without dust settling on the surface.

The thickness of the wax coating 8 is somewhat more critical. In general. a thickness of less than 1 micron should be used for making screens for camera tubes. It is. of course, obvious that a thick coating of wax would completely mask the fine grooves and prevent any subsequent electro-deposition in the form of a grid work of lines. For this reason. it is apparent that the thickness of the wax coating should be less than the depth of the grooves.

The solvent is evaporated from the wax coating by exposure to the atmosphere and the master is next given a thin coating 10 of sputtered metal over the entire waxcoated surface. The master plate is placed inside a conventional'evacuable bell jar for the sputtering operation. The sputtering operation is carried out using a cathode of solid metal palladium. Although the solid palladium electrode is preferred, a cathode coated with a suspension of metal having a composition one part bismuth, seven parts palladium. and twenty-five parts gold may be used. With the exception of ditference in the preferred sputtering metal. thesputtering process is substantially the same as that described in the previously referred to U. S. Patent 2.529.086. The wax coated surface of the glass plates placed two to four inches from the cathode, the bellar is evacuated to a pressure of 0.1 mm. of mercury (air atmosphere) and a glow discharge is created at about 2500 volts and about 60 ma. These conditions are not 'filaments joined together at their intersections.

critical and will vary greatly depending on the size of the apparatus and the area to be sputtered. For example. the pressure inside the bell-jar may vary from about 0.03 to 0.3 mm. and sputtering voltages of about 1000 to 2500 volts and currents of about to 70 ma. may be used.

The sputtering process is carried on for about four minutes although this maybe varied considerably. Usually. sputtering times of 2 to 6 minutes are sufficient. In the present example. the thickness of the palladium coating is such that the transmission of white light through the glass plate is reduced from 85%, before s uttering starts to 40%. after sputternig is com lete. The thickness of the coating is so small that it is difficult to measure by usual methods. The coating thickness is just enough to provide an electrically conducting film over the wax. Like the wax coating, althou h the thickness may vary, it must be less than the depth of the grooves.

The glass master plate which has now been provided with a thin coatin of wax 8 and a thin coatin of sputtered palladium 10. is removed from the bell-jar and placed in a dish of distilled water. While submer ed in the distilled water, the coated surface of the plate is rubbed gently so as to remove all of the palladium metal from the raised areas 12 between the grooves (see Fi ure 6) leaving only those portions of the metal 10' which have been de osited in the rooves on top of the wax. In this step of the process, the wax coating in the raised areas between the grooves is also partially removed althou h whether this is removed or not does not matter.

After the rubbing step, the plate is rinsed with water and placed in a plating bath in order to have a further thickness of metal 14 deposited in the grooves. The plating bath may be one suitable for depositing nickel. and this may consist essentially of 280 grams per liter of nickel sulfate having a composition NiSO4.7H2O, 180 grams per liter of nickel ch oride having the composition NiClv.6HzO, 35 grams per liter of boric acid and enough sulphuric acid to adiust the pH of the solution to 4.0. The pH of the solution should be kept between about 4.0 and 5.0 during the plating operation for best results.

In=this bath the coated plate is subjected to the electrodeposition of metal for about eight minutes at a current densitv of .16 amp. per square inch. When the electro-' deposition is complete, the master plate will now have a deposit of nickel 14 in all of the grooves on top of the palladium film. The plate is next rinsed in distilled water and the metal screen 16 is floated free of the glass plate. When the screen is made of electro-deposited nickel, the palladium metal usually stays on the nickel so that the screen separates from the master plate at the surface of.

from the wax without any tearing and, for some reason not understood, the screen develops free of strain and does not have nearly as much tendency to curl and tear as do screens using previously used processes.

As shown in Figure 8, the completed screen 16, after removal from the glass plate, is a grid work of m trtilill e screen is flushed with acetone and is dried by placing between two sheets of bond paper in a current of clean dry air. For rapid drying, the air may be somewhat above room temperature.

A 720 mesh screen made by this process has from about 40%85% light transmission value-depending upon the width of the grooves selected and the amount of metal electro-deposited to form the screen. In the example described, the screen has a light transmission of about 50%.

Although nickel has been given as the electro-deposited metal in the above example. any metal capable of forminga coherent deposit under electro-deposition can be used. Other suitable metals are copper, silver, zinc, and platinum, but it is to be understood that these are only given as examples. Compositions of the plating bath are not critical. bath may be made by dissolving 300 grams of copper sulfate (CuSOaSHzO) per liter of water and adiusting the pH to a value of 1.0 by adding sulphuric acid. A screen may be formed in this bath by electroplating using a current density of 0.2 amp. per square inch.

For the sputtering metal, gold or platinum may be used although palladium is preferred.

There has thus been described an improved method of making fine mesh screens for any desired purpose where very thin screens having relatively high light or electron transmission are required. The screens, themselves, are

much improved in quality because of the presence of which comprises in combination the steps of applying to a ceramic master plate ruled with a network of grooves a solution of wax suspended in a volatile solvent. evaporating said solvent and thereby forming a thin film of wax on the surface of said plate the thickness of said wax layer being less than the depth of said grooves. sputtering onto the surface of said wax a film of at least one metal selected from the group consisting of gold. platinum and palladiumin a vacuum by an electrical discharge from a solid electrode of said metal. removing substantially all of said wax and sputtered metal only from the elevated areas of said plate. electrodeoositing additional metal over the sputtered metal remaining in said grooves, and separating the formed screen from said master late.

2. A method of making a fine mesh metallic screen which comprises in combination the steps of applying to a glass master plate ruled with a network of horizontal 1 and vertical grooves, there being about 720 grooves per inch in each direction, a solution of beeswax suspended in petroleum ether, evaporating said solvent and thereby forming a thin film of beeswax on the surface of said plate, the thickness of said wax layer being less than the depth of said grooves, sputtering'onto the surface of said wax a film of palladium, in a vacuum by an electrical discharge from a solid electrode of said metal, removing substantially all of said wax and sputtered metal only from the elevated areas of said plate, electrodepositing nickel metal over the sputtered metal remaining in said grooves, and separating the formed screen from said master plate.

3. The method of claim 1 in which said sputtered metal is palladium.

4. The method of claim 1 in which said sputtered metal is gold.

5. The method of claim 1 in which the number of said grooves is at least 500 per inch.

6. The method of claim 1 in which said sputtered metal coating is platinum.

7. The method 'of claim 1 in which said electro-dee posited metal is nickel.

If copper is the plating metal, a suitable References Cited in the file of this patent 5 634,376 UNITED STATES PATENTS Callow Mar. 10, 1885 Law Nov. 7, 1950 6 FOREIGN PATENTS Great Britain 1891 Great Britain Jan. 25, 1937 Great Britain Mar. 22, 1950 

1. A METHOD OF MAKING A FINE MESH METALLIC SCREEN WHICH COMPRISES IN COMBINATION THE STEPS OF APPLYING TO A CERAMIC MASTER PLATE RULED WITH A NETWORK OF GROOVES A SOLUTION OF WAX SUSPENDED IN A VOLATILE SOLVENT, EVAPORATING SAID SOLVENT AND TEHREBY FORMING A THIN FILM OF WAX ON THE SURFACE OF SAID PLATE THE THICKNESS OF SAID WAX LAYER BEING LESS THAN THE DEPTH OF SAID GROOVES, SPUTTERING ONTO THE SURFACE OF SAID WAX A FILM OF AT LEAST ONE METAL SELECTED FROM THE GROUP CONSISTING OF GOLD, PLATINUM AND PALLADIUM IN A VACUUM BY AN ELECTRICAL DISCHARGE FROM A SOLID ELECTRODE OF SAID METAL, REMOVING SUBSTANTIALLY ALL OF SAID WAX AND SPUTTERED METAL ONLY FROM THE ELEVATED AREAS OF SAID PLATE, ELECTRODEPOSITION ADDITIONAL METAL OVER THE SPUTTERED METAL REMAINING IN SAID GROOVES, AND SEPARATING THE FORMED SCREEN FROM SAID MASTER PLATE. 